![]() With a zero signal applied to the Base of the transistor it turns “OFF” acting like an open switch and zero collector current flows. Then the transistor operates as a “single-pole single-throw” (SPST) solid state switch. For a PNP transistor, the Emitter potential must be positive with respect to the Base. Then we can define the “saturation region” or “ON mode” when using a bipolar transistor as a switch as being, both junctions forward biased, V B > 0.7v and I C = Maximum. This means then that we can ignore the operating Q-point biasing and voltage divider circuitry required for amplification, and use the transistor as a switch by driving it back and forth between its “fully-OFF” (cut-off) and “fully-ON” (saturation) regions as shown below. The areas of operation for a transistor switch are known as the Saturation Region and the Cut-off Region. If the circuit uses the Bipolar Transistor as a Switch, then the biasing of the transistor, either NPN or PNP is arranged to operate the transistor at both sides of the ” I-V ” characteristics curves we have seen previously. However, high power devices such as motors, solenoids or lamps, often require more power than that supplied by an ordinary logic gate so transistor switches are used. Some output devices, such as LED’s only require a few milliamps at logic level DC voltages and can therefore be driven directly by the output of a logic gate. ![]() Solid state switches are one of the main applications for the use of transistor to switch a DC output “ON” or “OFF”. However, both the NPN & PNP type bipolar transistors can be made to operate as “ON/OFF” type solid state switch by biasing the transistors Base terminal differently operating the transistor as a switch. Typical bipolar transistors must be connected in the normal direction, except for muting transistors * that are designed for reverse connection.When used as an AC signal amplifier, the transistors Base biasing voltage is applied in such a way that it always operates within its “active” region, that is the linear part of the output characteristics curves are used. As a result of the foregoing, a reverse-connected transistor might be permanently damaged. Moreover, because of the decreased V CEO, the transistor might break down when it turns off. In this case, h FE decreases, making it impossible for a bipolar transistor to function as intended and possibly causing the base current to exceed its specified rating. If the connections of the collector and emitter terminals are reversed, the above relationship is not satisfied. Therefore, the dopant concentrations in the three semiconductor regions have the following relationship: emitter > base > collector. reducing the dopant concentration in the collector region.reducing the thickness of the base region.making the dopant concentration in the emitter region higher than that in the base region.It is also necessary to control the depletion layer in the collector region in order to increase a transistor’s withstand voltage (i.e., collector-base voltage, V CBO). To accomplish this, numerous majority carriers (electrons in the case of npn transistors) in the emitter must diffuse into the base region and then be efficiently swept across the base region into the collector. Generally, bipolar transistors are designed in such a manner as to achieve high h FE. Interchanging the collector and emitter terminals not only degrades the transistor performance but also might cause permanent damage to the device. Transistors are designed to provide the optimum performance when they are correctly connected.
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